EOT

EOT was created by Microsoft in 1997 as the first web font format ever. It was submitted to the W3C for standardization but was never accepted — largely because of its proprietary compression algorithm (MTX) and built-in DRM features that other browser vendors rejected.

The format includes URL binding, a mechanism that restricts fonts to work only on specific domains, and supports font subsetting to reduce file size by stripping unused glyphs. These were forward-thinking features for the late 1990s, but the proprietary nature of the format meant it was only ever implemented in Internet Explorer.

For nearly a decade — from IE 4 through to the mid-2000s — EOT was the only way to use custom fonts on the web. It wasn't until WOFF arrived in 2010 that a cross-browser alternative existed. Today, EOT is a relic: useful only for the rare project that must still support IE 8 or earlier.

EOT files wrap TrueType font data with Microsoft's MTX compression and a proprietary header. The header contains URL binding information (a list of domains where the font is permitted to load), licensing flags, and font metadata such as family name and style.

When Internet Explorer encounters an EOT font in a stylesheet, it reads the header, verifies that the current page's origin URL matches one of the permitted domains, and only then decompresses and renders the font. If the URL check fails, the font is silently rejected — a basic form of hotlink protection.

The classic "bulletproof" @font-face syntax that includes EOT looks like this:

@font-face {
  font-family: 'MyIcons';
  src: url('icons.eot');                    /* IE9 compat */
  src: url('icons.eot?#iefix') format('embedded-opentype'),
       url('icons.woff2') format('woff2'),
       url('icons.woff') format('woff'),
       url('icons.ttf') format('truetype');
}

The ?#iefix query string is a hack to work around an IE 9 parser bug. IE 9 couldn't correctly handle multiple src entries in a single declaration — it would try to load the entire string (including the format() hints) as a single URL, which would fail. Adding ?#iefix to the EOT URL tricks IE 9 into treating it as a valid, standalone URL by terminating the path before the parser chokes on the rest of the declaration. Modern browsers ignore the query string entirely.

Only when you must support IE 8 or earlier. These ancient browsers don't understand WOFF, WOFF2, or even raw TTF via @font-face — EOT is their only option.

For IE 9-11, WOFF is the better choice: it's an open standard, has broader tool support, and doesn't carry the baggage of DRM and URL binding. If your analytics show zero IE 8 traffic — which is overwhelmingly likely in 2024 and beyond — skip EOT entirely.

Before including EOT in your build, check your actual browser analytics. The global usage share of IE 8 and below is effectively 0%. Unless you're targeting a specific enterprise intranet or government system locked to an ancient IE version, EOT adds build complexity for no practical benefit.